1. Field of the Invention
The present invention relates to NOx reduction, more particularly to reduction of NOx by NOx trap technology, and more particularly to systems for decomposing NOx to N2 and other benign gases in oxygen-rich environments.
2. Description of Related Art
The control of NOx emissions from vehicles is a worldwide environmental problem. Gasoline engine vehicles can use newly developed three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called xe2x80x9clean-burnxe2x80x9d gas engines, and diesel engines too, have so much oxygen in their exhausts that conventional catalytic systems are effectively disabled. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn engine exhausts can inhibit NOx removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NOx emissions under net oxidizing conditions is also critical for diesel engines.
Catalysts that promote the reduction of NOx under oxygen-rich conditions are generally known as lean-NOx catalysts. Difficulty has been encountered in finding lean-NOx catalysts that have the activity, durability, and temperature window required to effectively remove NOx from the exhaust of lean-burn engines. Prior art lean-NOx catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges.
Such catalysts that can effectively decompose NOx to N2 and O2 in oxygen-rich environments have been the subject of considerable research. (For instance, see, U.S. Pat. No. 5,208,205, issued May 4, 1993, to Subramanian, et al.) One alternative is to use catalysts that selectively reduce NOx in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia as a co-reductant.
However, another viable alternative involves using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline engines as a co-reductant and is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable SCR catalysts that work with hydrocarbon co-reductants in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
In the presence of hydrocarbons, catalysts that selectively promote the reduction of NOx under oxygen-rich conditions are known as lean-NOx catalysts, and more specificallyxe2x80x94SCR lean-NOx catalysts. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NOx to N2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NOx catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SOx). As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NOx catalysts must overcome all three problems simultaneously before they can be considered for commercial use.
In the case of sulfur poisoning, some gasoline can contain up to 1200 ppm of organo-sulfur compounds. Lean-NOx catalysts promote the conversion of such compounds to SO2 and SO3 during combustion. Such SO2 will adsorb onto the precious metal sites at temperatures below 300xc2x0 C. and thereby inhibits the catalytic conversions of CO, Cx Hy (hydrocarbons) and NOx. At higher temperatures with an Al2O3 catalyst carrier, SO2 is converted to SO3 to form a large-volume, low-density material, Al2(SO4)3, that alters the catalyst surface area and leads to deactivation. In the prior art, the primary solution to this problem has been to use fuels with low sulfur contents.
A second leading catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, commonly called the xe2x80x9clean-NOx trap.xe2x80x9d As with SCR lean-NOx catalysts, the lean-NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals; however, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus xe2x80x9ctrappedxe2x80x9d, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoiciometrically fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
Both lean-NOx SCR and lean-NOx-trap technologies have been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO2 are also active in converting SO2 to SO3. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SOx adsorbs more strongly on NO2 adsorption sites than NO2, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. Such presence of SO3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed, for example, Nakatsuji et al. describing the use of selective SOx adsorbents upstream of lean NOx trap adsorbents.
Furthermore, catalytic oxidation of NO to NO2 is limited in its temperature range. Oxidation of NO to NO2 by a conventional Pt-based catalyst maximizes at about 250xc2x0 C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NOx trap technology with respect to temperature and sulfur considerations.
The U.S. Federal Test Procedure for cold starting gasoline fueled vehicles presents a big challenge for lean-NOx trap catalysts due to the low-temperature operation involved. Diesel passenger car applications are similarly challenged by the driving cycle that simulates slow-moving traffic. Both tests require reductions of CO, hydrocarbons, and NOx at temperatures below 200xc2x0 C. when located in the under-floor position. Modifications of existing catalyst oxidation technology are successfully being used to address the problem of CO and hydrocarbon emissions, but a need still exists for improved NOx removal.
The present invention provides a method for reducing NOx emissions and a vehicle with reduced NOx emissions. The present invention also provides a system for attachment to an engine with an oxygen rich exhaust for the reduction of NOx emissions.
Briefly, the present invention comprises a non-thermal plasma gas treatment of NO to produce NO2 which is then combined with catalytic storage reduction treatment, e.g., a lean NOx trap, to enhance NOx reduction in oxygen-rich vehicle engine exhausts. In the lean NOx trap of the invention, the NO2 from the plasma treatment is adsorbed on a nitrate-forming material, such as an alkali material, and stored as a nitrate. An engine controller periodically runs a brief fuel-rich condition to provide hydrocarbons for a reaction that decomposes the stored nitrate into benign products such as N2. By using a plasma, the lean NOx trap catalyst can be implemented with known NOx adsorbers, and the catalyst may contain less or essentially no precious metals, such as Pt, Pd and Rh, for reduction of the nitrate to N2.
Accordingly, an advantage of the present invention is that a method for NOx emission reduction is provided that is inexpensive and reliable. The plasma-assisted lean NOx trap can allow the life of precious metal lean NOx trap catalysts to be extended for relatively inexpensive compliance to NOx emission reduction laws.
Furthermore, not only does the plasma-assisted lean NOx trap process improve the activity, durability, and temperature window of lean NOx trap catalysis, but it also allows the combustion of fuels containing relatively high sulfur contents with a concommitant reduction of NOx, particularly in an oxygen-rich vehicular environment. The present invention allows the use of a lean NOx trap to reduce NOx emissions in engine exhausts containing relatively high concentrations of sulfur, such as greater than 20 ppmw sulfur (calculated as S).